1. Field of the Invention
The present disclosure generally relates to the field of integrated circuit manufacture and, more particularly, to a method and a system for optimizing a wafer layout.
2. Description of the Related Art
The formation of various integrated circuit (IC) structures on a wafer usually involves lithographic processes, sometimes referred to as photolithography or simply lithography. As is well known, lithographic processors can be used to transfer a pattern of a photomask (also referred to herein as a mask or a reticle) to a wafer. For instance, a pattern may be formed from a photoresist layer disposed on the wafer by passing light energy through a reticle having an arrangement to image the desired pattern of the respective level of the integrated circuit onto the photoresist layer. As a result, the pattern is transferred to the photoresist layer. Usually, after exposing a photoresist layer, a development cycle is performed, e.g., via an appropriate heat processing of the wafer. With some types of photoresist material, i.e., in areas where the photoresist is sufficiently exposed, the photoresist material may become soluble such that it may be selectively removed to expose an underlying layer, e.g., a semi-conductor layer, a metal or metal-containing layer, a dielectric layer, a hardmask layer, etc. Portions of the photoresist layer not exposed to a threshold amount of light energy will not be removed and serve to protect the underlying layer during further processing of the wafer. Further processing of the wafer may include, but is not limited to, etching exposed portions of the underlying layer, implanting ions into the wafer, etc. Thereafter, the remaining (protecting) portions of the photoresist layer may be removed.
There is an ongoing trend in IC fabrication to increase the density of the structures of the IC and, in particular, the density of the individual elements. As a result, there is a corresponding need to increase the resolution capability of lithography systems. Generally, the resolution of the lithography system is limited at least in part by the wave length used to expose portions of the photoresist. In particular, the finest resolvable or critical dimension is proportional to the wavelength of the light used for exposure. Another limiting factor is the numerical aperture, the critical dimension being inversely proportional to the numerical aperture. Accordingly, there has been a trend to reduce the wavelength as well as to increase the numerical aperture.
In modern systems, the reticle is not directly projected onto the wafer, but rather an imaging system is used to project the exposure pattern onto the wafer. Such an imaging system is capable of reducing the size of the structure defined by the reticle by a certain factor, e.g., a factor of 4 to 6. However, the imaging system cannot overcome the limitations imposed by the wavelength and the numerical aperture. Therefore, in order to further increase the resolution of the lithographic system, as an alternative to conventional dry lithography systems, an immersion lithography system has been proposed wherein the light is not transmitted through air or a vacuum from the imaging system to the wafer, but rather through an immersion lithography medium. Such an immersion lithography medium may be purified and de-ionized water. The immersion medium may be selected depending on the wavelength used for exposure. The immersion medium replaces an air or gas gap that is conventionally present between the final optical component of a conventional dry lithography imaging system and the wafer.
Usually, a wafer contains a plurality of individual devices (die). In order to increase the resolution and the quality of the exposure of the photoresist, a reticle is usually not configured for exposure of the whole wafer, but rather for exposure of only a part of the wafer, a so-called flash field or exposure field. For example, the reticle may include the structure of one device or a few devices, e.g., two or four devices. This means that, for exposing the whole wafer, the imaging system and the wafer have to be moved with respect to each other in order to expose the whole wafer step by step. Accordingly, systems of this type are usually referred to as steppers. Some steppers expose the whole reticle in one shot, whereas other types of lithographic systems only expose the whole width of the reticle at one time and exposure of the whole reticle is done by scanning the reticle in a direction perpendicular to its width.
One influence factor for cost efficiency in semiconductor manufacturing is the product yield. On the one hand, the quality of every single production step should be ideally set up getting the maximum yield out of the wafer. On the other hand, the total number of die that are positioned on one wafer is another important factor. Parameters which may influence this number of die are, for example, the wafer diameter, the wafer edge clearance, a shot clearance, the die dimension and the wafer layout itself, i.e., the arrangement of the die over the wafer.
The present disclosure is directed to various methods and systems that may avoid, or at least reduce, the effects of one or more of the problems identified above.